The present invention is directed to a process for the selective production of pseudocumene by catalytic methylation of xylene in the presence of a solid catalyst.
Xylenes and higher aromatics such as pseudocumene and mesitylene are desired commercial products having various uses as disclosed in N. E. Ockerbloom's article, "Xylenes and Higher Aromatics" in Hydrocarbon Processing, April 1972, pg. 114-118. This article discloses that these products can be obtained through fractional distillation or extraction from C.sub.9 aromatic fractions obtained from naphtha cracking or reformate. Pseudocumene may be oxidized to form trimellitic acid which is useful in the manufacture of synthetic fibers and plastics. It may also be desirable to manufacture the acid anhydride form. Thus, there have been efforts to devise processes resulting in a high purity pseudocumene product. Fractionation of an extracted, heavy catalytic reformate containing about 40% pseudocumene to obtain purified pseudocumene requires large fractionation towers to perform the separation. Using a crystalline zeolite catalyst on an extracted reformate cut to increase pseudocumene content has been proposed in U.S. Pat. No. 5,004,854 to Yan.
The term "shape-selective catalysis" describes the catalytic selectivities found in molecular sieves such as zeolites. The principles behind shape selective catalysis have been reviewed extensively, e.g., by N. Y. Chen, W. E. Garwood and F. G. Dwyer, Shape Selective Catalysis in Industrial Applications, 36, Marcel Dekker, Inc. Reactant selectivity occurs when a fraction of the feedstock is too large to enter the zeolite pores to react; while product selectivity occurs when some of the products cannot leave the zeolite channels. Product distributions can also be altered by transition state selectivity in which certain reactions cannot occur because the reaction transition state is too large to form within zeolite pores or cages. Another type of selectivity results from configurational restraints on diffusion where the dimensions of the molecule approach that of the zeolite pore system. A small change in the dimensions of the molecule or the zeolite pore can result in large diffusion changes leading to different product distributions.
Several methods have been proposed for the alkylation of aromatic compounds with methanol to produce xylenes, or higher molecular weight aromatics such as trimethylbenzenes or tetramethylbenzenes.
The alkylation of toluene with methanol over cation-exchanged zeolite Y has been described by Yashima et al. in the Journal of Catalysis 16, 273-280 (1970). The selective production of para-xylene was reported over the approximate temperature range of 200 to 275.degree. C., with the maximum yield of para-xylene being observed at 225.degree. C. Higher temperatures were reported to result in a decrease in production of para and ortho-xylenes.
U.S. Pat. No. 3,965,209 to Butter et al. and U.S. Pat. No. 4,067,920 to Kaeding disclose processes for producing para-xylene in low conversion and high selectivity by reaction of toluene with methanol over a zeolite such as ZSM-5. In Butter et al the zeolite is steamed at a temperature of 250-1000.degree. C. for 0.5-100 hours to reduce the acid activity of the zeolite.
U.S. Pat. No. 4,001,346 to Chu relates to a process for the selective production of para-xylene by methylation of toluene in the presence of a catalyst comprising a crystalline aluminosilicate zeolite which has undergone prior treatment to deposit a coating of between about 15 and about 75 wt. % of coke thereon.
U.S. Pat. No. 4,380,685 to Chu relates to the shape selective reactions carried out with zeolite catalysts modified with iron and/or cobalt, and the alkylation of toluene and xylene at temperatures up to 750.degree. C. is disclosed. A zeolite such as ZSM-5 is preferred, and optionally the catalyst is further modified by the incorporation of phosphorus and/or by steaming at a temperature of 250-1000.degree. C.
A process for the methylation of toluene to selectively produce para-xylene is disclosed in International Publication Number WO 98/14415. The process employs a catalyst having a Diffusion Parameter for 2,2-dimethylbutane of about 0.1-15 sec-1. It is disclosed that the required diffisivity for the catalyst can be achieved by severely steaming the catalyst so as to effect a controlled reduction in the micropore volume of the catalyst to not less than 50%, and preferably 50-90%, of that of the unsteamed catalyst.
A process for the selective production of pseudocumene and durene in higher than equilibrium concentrations by methylation of benzene or methyl-substituted benzenes in the presence of AMS-1B crystalline borosilicate at temperatures up to 1000.degree. F. is disclosed in U.S. Pat. No. 4,891,467 to Sikkenga. However, the process produces substantial quantities of durene, resulting in a pseudocumene:durene ratios less than 8:1 in the final product stream.
Another process for the production of pseudocumene over a magnesium modified AMS-1B catalyst is disclosed in U.S. Pat. No. 4,665,254 to De Simone wherein methylation of xylene over the hydrogen form of the catalyst is reported to yield a trimethylbenzene fraction with a high selectivity to pseudocumene. However, pseudocumene selectivity for the patented process decreases both with increased temperature and with a mixed xylenes feed.
It would be desirable to produce a high purity pseudocumene product via a xylene methylation process amenable to a variety of xylene feeds, and operating at high xylene conversions with high utilization of the methanol feedstock to convert xylene into the pseudocumene product. Ideally, such a xylene methylation process would selectively produce 1,2,4 trimethylbenzene (pseudocumene) in lieu of other trimethylbenzene isomers and would also have a high pseudocumene:durene ratio, i.e., the further methylation of trimethylbenzene to tetramethylbenzene would be insubstantial. Thus, the need for complex purification steps to increase the pseudocumene content of the product would be avoided.